1. Field of the Invention
The present invention relates to a powder transfer device, and to a process for producing polyolefins using such a device.
2. Related Background Art
A tube called a “downcomer” that communicates between an upper stage and a lower stage has hitherto been known as a means for transferring powder from an upper stage to a lower stage in an apparatus such as a vertical multistage fluidized bed reactor (see, for example, U.S. Pat. No. 5,235,009).
With the use of such a downcomer, powder in an upper stage which has dropped into a downcomer from an opening in the top end of the downcomer falls gravitationally through the downcomer and is discharged from an opening at the bottom end of the downcomer, thereby enabling powder to be transferred from the upper stage to the lower stage.
However, in prior-art downcomers, the powder transfer rate cannot be controlled as desired. That is, the rate of powder transfer is determined primarily by such mechanical shape factors as the diameter of the downcomer and the positions of the top and bottom ends of the downcomer. Controlling the powder transfer rate has been difficult.
While it is conceivable to provide a mechanical valve on the downcomer for the purpose of controlling the flow rate of powder through the downcomer, installing a mechanical valve having a complicated structure within the device is undesirable because it will cause various problems.